Phloem tissue transports sugars around a plant. Explain how the structure of phloem tissue is suited to its function of transporting sugars from leaves to other parts of the plant.

Pearson Edexcel A-Level Biology A (9BI0) — 2.4 Plant structure and function · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Phloem is a specialised plant tissue found throughout the plant body. It transports dissolved sugars, produced during photosynthesis in the leaves, to areas such as roots and growing shoots where the sugars are needed for respiration and growth.

Model answer (5 marks)

1. Phloem transports sugars (dissolved food) from the leaves (source) to other parts of the plant such as roots or growing regions (sink). 2. Sieve tube elements have sieve plates or pores at the end walls, allowing sugars to flow through between cells. 3. Sieve tube elements have reduced or no nucleus and few organelles, leaving more space for the flow of sugars. 4. Companion cells have many mitochondria to provide energy (ATP) to actively load sugars into the phloem and maintain the sieve tube elements. 5. Phloem forms a continuous network of tubes throughout the plant, allowing sugars to reach all parts including roots and growing shoot tips.

Examiner tips

  • Use the exact terms: source, sink, sieve tube elements, companion cells, ATP. 2–3 points per sentence to hit 5 marks. 5. Include the continuous network point to show full understanding.

Common mistakes

  • Confusing sieve tube elements with phloem fibres. 2. Forgetting the role of companion cells and ATP. 3. Mixing up xylem and phloem functions.

Mark scheme (5 marks)

  1. Phloem transports sugars (dissolved food / assimilates) from the leaves (source) to other parts of the plant such as roots or growing regions (sink)
  2. Phloem sieve tube elements have sieve plates / pores at the end walls, which allow sugars to flow through between cells
  3. Sieve tube elements have reduced / no nucleus and few organelles, leaving more space for the flow of sugars
  4. Companion cells have many mitochondria to provide energy (ATP) to actively load sugars into the phloem / maintain the sieve tube elements
  5. Phloem forms a continuous network / tubes throughout the plant, allowing sugars to reach all parts including roots and growing shoot tips

Key terms in this question

phloem

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